Welding output prevention control having open condition detection

ABSTRACT

A method and apparatus for welding is disclosed. It includes source of power and a controller. An output feedback circuit provides feedback to the controller. The controller includes a comparator that compares the fedback signal to a threshold and/or detects a short on the output. A standby/welding control is responsive to the comparator. Also, an arc end control may be provided that terminates the arc in response to detecting an increase in arc length.

FIELD OF THE INVENTION

[0001] The present invention relates generally to the art of welding.More specifically, it relates to providing a reduced OCV and forterminating the arc at the end of a weld.

BACKGROUND OF THE INVENTION

[0002] There are a number of different welding processes. Two knownwelding processes are stick welding and TIG. Each welding process hasits own characteristics, objectives and problems that often make itdifficult to adapt a control scheme from one process to be used withanother process.

[0003] Stick welding (also called SMAW or Shielded Metal Arc Welding) isan arc welding process which uses a consumable stick electrode, and maybe performed using an ac or a dc output. The stick welding process isoften touch or scratch started, wherein 80 volts (typically) is the opencircuit voltage (OCV). The user scratches or touches the electrode tothe workpiece and the OCV causes current to flow. The user pulls theelectrode away from the workpiece, drawing an arc therebetween. Thisstarting process may be adequate for skilled welders, but is oftendifficult for less experienced welders. Also, 80 volts OCV may besufficient for a user to feel an electric shock if they touch theelectrode and the workpiece, or the output studs of the power supply.While this might not injure the worker, it can result in downtime whilethe worker receives attention making sure they are not injured, and/orthe power supply is serviced.

[0004] Tungsten-inert gas (TIG) welding is a welding process where atungsten electrode is used, and the electrode does not become part ofthe completed weld. One known way of starting the TIG welding process isthe Miller LiftArc®, which provides a reduced OCV in a standby state,and then monitors the output to detect a short-circuit between theelectrode and the workpiece. A detected short (which occurs when theuser touches the electrode to the workpiece) indicates the users intentto start the weld. The power supply then enters a preheating state,where a low current is provided to heat the electrode. After theelectrode is heated the user needs to lift the electrode away from theworkpiece a second time to cause the power supply to enters a weldingstate where the desired welding current is provided. Such a startingscheme will not function properly in stick welding because the stickwill adhere to the workpiece after preheating.

[0005] Accordingly, a welding power supply that allows for easy startingof a stick welding process, yet provides for a reduced OCV when notwelding is desirable. Also, the power supply should provide a start thatdoes not cause the electrode to adhere to the workpiece.

[0006] Other problems occur when the arc welding process is terminated.The simplest way to terminate a weld is to pull the electrode away fromthe workpiece, drawing a longer arc. The machine output voltageincreases, in an attempt to maintain the arc. Eventually, the arc lengthis so great that the power supply cannot provide voltage sufficient tomaintain the arc, and the arc is extinguished. One problem withterminating the arc in this fashion is that when the arc is relativelylong it has a tendency to whip across the surface of the workpiece,leaving undesired weld tracks. This problem is particularly prevalent inending a TIG process.

[0007] Another known way to terminate a welding process is to provide aremote switch that the user can reach while welding. The switchterminates the weld process abruptly. It is remote so that the user canaccess it without taking his attention from the arc, lest the arc strayacross the workpiece. Unfortunately, a remote control adds cost andcomplexity to a welding power supply.

[0008] A known way to terminate a stick welding process is to monitorthe current in an inverter in the power supply, and to terminate powerwhen the current crosses below a threshold. However, this process is notwell suited for TIG because the arc can become undesirably long beforethe current drops below a threshold that avoids false detection of theend of the arc. Thus, the arc can whip across the workpiece.

[0009] Accordingly, a welding power supply that ends a TIG weldingprocess easily, cleanly, and without adding excess cost is desirable.

SUMMARY OF THE PRESENT INVENTION

[0010] According to a first aspect of the invention a welding powersupply includes a source of power and a controller. An output feedbackcircuit provides feedback to the controller. The controller includes acomparator that compares the fedback signal to a threshold. Astandby/welding control is responsive to the comparator.

[0011] According to a second aspect of the invention an apparatus forstarting a welding process includes an output feedback circuit and ashort-circuit detect circuit, that receives the fedback signal. Acontroller provides a standby state output to a power supply if theelectrode is not initially touching the workpiece. It provides a weldingstate output if the electrode subsequently touches the workpiece.

[0012] According to a third aspect of the invention a welding powersupply includes a source of power and a controller for the source ofpower. An output feedback circuit provides feedback to the controller,The controller includes a comparator that receives a signal indicativeof a threshold and the feedback signal. The controller further includesan arc end control that responds to the comparator.

[0013] The controls include at least a portion of a digital circuit, andis a microprocessor with instruction in various embodiments. Theinstructions control the output voltage to a first level an initial timethe comparator has a first output. Then, they control the output voltageto a second level when the comparator has a second output in anotherembodiment. The instructions control the output of the power supply to awelding status in the event the comparator has a third output and thatcontrol the output of the power supply to an arc end status in the eventthe comparator has a fourth output for a period of time.

[0014] The feedback circuit includes a voltage feedback circuit and/or acurrent feedback circuit in alternative embodiments.

[0015] The comparator is a delay comparator in one embodiment.

[0016] According to a fourth aspect of the invention a method ofproviding welding power includes sensing an output parameter andcomparing the sensed parameter to a threshold. A power supply iscontrolled to be in a standby or welding state in response to thecomparison.

[0017] According to a fifth aspect of the invention a method ofproviding welding power includes sensing at least one output parameterand comparing it to a threshold. A power supply is controlled to be in awelding state or an arc end state in response to the comparison.

[0018] According to a sixth aspect of the invention a method of ending awelding process includes sensing an output parameter and determining ifan arc length exceeds a threshold. A power supply is controlled to be ina welding state if the arc length does not exceed the threshold, or inan arc end state if the arc length does exceed the threshold.

[0019] The output voltage is controlled to a first level or a secondlevel in response to the comparison in one alternative.

[0020] The sensed output parameter is voltage and/or current, and thecomparison is output voltage and/or current to the threshold in otheralternatives.

[0021] The welding state includes a first output voltage and the standbystate includes terminating the output after a delay in one embodiment.

[0022] According to a seventh aspect of the invention a method ofstarting a welding process includes sensing an output parameter, anddetermining if an electrode touches a workpiece in response to thesensed parameter. A power supply is controlled to be in a standby stateif the electrode is initially not touching the workpiece, and thencontrolled to be in a welding state if the electrode is touching theworkpiece.

[0023] According to an eighth aspect of the invention an apparatus forending a welding process includes an output feedback circuit and an arclength detect circuit. A controller provides a welding state output ifthe arc length is less than a threshold and provides an arc end stateoutput if the arc length is greater than the threshold.

[0024] The welding state includes a first greater output voltage and thestandby state includes a second lessor output voltage in an alternative,and controlling the output voltage to a first level or terminating theoutput in another alternative.

[0025] The sensed parameter is output voltage or current, which is thencompared to a threshold in other alternatives.

[0026] Other principal features and advantages of the invention willbecome apparent to those skilled in the art upon review of the followingdrawings, the detailed description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a block diagram representing functions of a controllerof a welding power supply in accordance with the present invention;

[0028]FIG. 2 is a block diagram of a welding power supply in accordancewith the present invention; and

[0029]FIG. 3 is a flow chart of a welding process in accordance with thepresent invention.

[0030] Before explaining at least one embodiment of the invention indetail it is to be understood that the invention is not limited in itsapplication to the details of construction and the arrangement of thecomponents set forth in the following description or illustrated in thedrawings. The invention is capable of other embodiments or of beingpracticed or carried out in various ways. Also, it is to be understoodthat the phraseology and terminology employed herein is for the purposeof description and should not be regarded as limiting. Like referencenumerals are used to indicate like components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] While the present invention will be illustrated with reference toa particular power supply, using particular components, to perform aparticular process, it should be understood at the outset that theinvention may also be implemented with other power supplies, componentsand/or processes.

[0032] Generally, the invention relates to a stick welding power supplythat provides a reduced output voltage and a desirable start. While itis often difficult to adopt a control scheme for one welding process toanother welding process, the particular controls described herein arereadily adaptable to other welding processes. The power supply is in astandby state initially, wherein a low OCV (such as 9-13V at the outputstuds in the preferred embodiment) is provided. The user touches theelectrode to the workpiece to signal the start of the welding process.The output voltage is monitored, and when it drops below a threshold (5Vin the preferred embodiment), the power supply is controlled to enter awelding state. Thus, a low OCV is provided until the user is ready toweld (and touches the electrode to the workpiece), and then the desiredwelding output is provided.

[0033] Standby state, as used herein, includes a state where the outputvoltage/and or current of the welding power supply is reduced, and/orfeatures relating to current, power, or voltage, such as boosts or usersetpoints are disabled, and/or the inverter portion of the power supplymay be turned off. Welding state, as used herein, includes a state wherethe output voltage and/or current of the welding power supply isprovided at the output set point such that welding can be performed,and/or features relating to current, power, or voltage, such as boostsor user setpoints, are enabled, and/or the inverter portion of the powersupply may be turned on, and excludes a preheat state.

[0034] The preferred embodiment is implemented using a Miller Maxstar®welding power supply, and generally includes the power supply describedin U.S. Pat. No. 6,115,273, issued Sep. 5, 2000, entitled PowerConverter With Low Loss Switching (hereby incorporated by reference),and assigned to the owner of this invention. Another power supply thatmay be used to implement this invention is found in the Miller XMT 304®,and is described in U.S. patent application Ser. No. 09/540,567, filedMar. 31, 2000, entitled Method And Apparatus For Receiving A UniversalInput Voltage In A Welding, Plasma Or Heating Power Source (herebyincorporated by reference), also owned by the owner of this invention.Of course, any welding power supply could be used to implement thisinvention.

[0035] A block diagram of a welding power supply constructed inaccordance with the preferred embodiment is shown in FIG. 2. Source 201represents the input voltage used to provide power to the welding powersupply, and may be from a utility source, battery, generator, or othersource. The input voltage is between 90 and 480 volts in the preferredembodiment, and is provided to a rectifier 202, which may be a simplebridge rectifier. The output of rectifier 202 is a rectified sinusoid. Apre-regulator 204 receives the rectified sinusoid from rectifier 102 andprovides a dc bus output to an output invertor 205. Pre-regulator 204,in the preferred embodiment is a soft-switched boost convertor whichprovides close to a unity power factor. Other convertor or invertorconfigurations may be used. Convertor 205 is preferably a half-bridge,transformer isolated, soft (or slow) switched invertor. Such an outputcircuit will is described in detail in U.S. Pat. No. 6,115,273. Outputconvertor 205 is a typical forward convertor or another type ofconvertor in other embodiments. Other alternatives include usingdifferent power sources or power topologies, omitting the pre-regulatoror the output converter, using a transformer based power supply, a phasecontrol based power supply, or any other type of power supply. Powersource, or source of power, as used herein, includes the power circuitrysuch as rectifiers, switches, transformers, SCRs, etc that process andprovide the output power.

[0036] Controller 210 receives an output feedback signal as an input.The output feedback signal may include voltage, current, power, orfunctions thereof (derivatives, integrals, etc.) Controller 210 alsoreceives signals from converter 205 and pre-regulator 204, and providescontrol signals thereto.

[0037] A block diagram representing some of functions of controller 210is shown in FIG. 1. Functions not shown may be performed as they are inthe prior art, or in other manners, without departing from thisinvention. Controller 210 includes a feedback circuit 101, a comparator103, a standby/welding control circuit 105 and a convertor on/offcontrol 107. Controller 210 includes a microprocessor in the preferredembodiment, and includes stored instructions (stored in flash memory,EPROM, code, software, firmware, etc.) It may be implemented usinganalog and/or digital circuitry, with discrete components and/orintegrated circuits such as microprocessors, DSPs, etc., in alternativeembodiments. Controller 210 may be located on a single board, ordistributed on a plurality of boards.

[0038] Feedback circuit 101 receives an output parameter, such ascurrent, voltage, power or functions thereof, and provides a signalresponsive thereto (i.e., the signal represents the fedback parameter(s)or function(s) thereof.) Feedback circuit 101 is implemented with amicroprocessor and software in the preferred embodiment, but may also beimplemented with analog and/or discrete components. Feedback circuit 101is an output feedback circuit since it receives an output parameter asthe fedback signal, but can more specifically be a voltage or currentfeedback circuit. Feedback circuit 101 senses the output parameterbecause it receives a signal indicative of the parameter. Voltage is thefedback parameter in the preferred embodiment.

[0039] Feedback circuit 101 provides the feedback signal to comparator103. Comparator 103 compares the fedback signal (voltage in thepreferred embodiment) to a threshold TH1. Comparator 101 is implementedwith software in the preferred embodiment, but may also be implementedwith analog and/or discrete components. (Comparator, as used herein,compares two or more input values and provides an output indicatingwhich of the input values is greater, and may be digital, analog or acombination thereof.)

[0040] The threshold is set at 5 volts in the preferred embodiment, andthe standby open circuit voltage is 13 volts. Thus, when the electrodetouches the workpiece the comparator output changes state (andcontroller 210 has determined the electrode touched the workpiece).Comparator 103 and feedback circuit 101 combine to detect shorts at theoutputs, and may also be called a short-circuit detect circuit.

[0041] The output of comparator 103 is provided to convertor on/offcontrol 107 which sends control signals to convertor 205. If a short ofthe output is not detected by comparator 203 then convertor 205 isdisabled, or inhibited from providing output current and voltage. (The13 volt output for the short detect is derived from the power supply forcontroller 210). Conversely, if a short of the output is detected bycomparator 203 then convertor 205 is enabled (preferably immediatelywithout intervening states such as preheating), and provides the weldingoutput selected by the user. Once the short has been detected comparator205 is locked into a state and converter 205 is enabled until the weldis over. Thus, the standby state is provided only for the initialcomparison (i.e. before the comparison changes) indicating no short.

[0042] The output of comparator 103 is also provided to astandby/welding control 105 which sends control signals to convertor205. If a short of the output is not detected by comparator 203 thencontrol 105 sends a standby state output control signal that causesconvertor 205 to be in a standby state, and not provide the userselected output, boost, etc. Conversely, if a short of the output isdetected by comparator 203 then control 105 sends a welding state outputcontrol signal that causes convertor 205 to be in a welding state, andprovide the user selected output, boost, etc. Thus, controller 210provides an output voltage control that sets the output voltage. Both ofcontrols 105 and 107 are used, or only one or the other are used, invarious embodiments.

[0043] A flow chart that implements the preferred welding processstarting routine is shown in FIG. 3. The process starts at step 300, andat step 302 the power supply is controlled to be in the standby state(having a reduced OCV and the converter disabled). If a short is notdetected at step 304, the process returns to step 302, and continues inthe standby state.

[0044] If a short is detected at step 304 the power supply is controlledto be in the welding state at step 306, with a full OCV, and controlledin accordance with the user setpoint (i.e., fully on).

[0045] The power supply continues on in the welding state until the userdesires to end the arc, by pulling the electrode away from theworkpiece. A longer arc results in an increased arc voltage and/ordecreased arc current. Thus, the arc current is monitored and adecreasing arc current indicates the user's intent to end the weldingprocess. The arc voltage and/or current is compared to a threshold, andthe derivative of the arc voltage and/or current is compared to athreshold in various embodiments. The arc current is compared to athreshold of 5 amps in the preferred embodiment, and a 50 msec time ofless than 5 amps indicates the end of the arc. The comparison ispreferably made by comparator 103, which receives current feedback and asecond threshold input TH2, but may be made by other circuitry,software, etc. Comparator 103, is called a delay comparator because itrequires that the threshold condition be met for a delay period of 50msec, in the preferred embodiment.

[0046] If the arc is not detected as ending at step 308, then theprocess continues in the welding state at step 306. However, if the arcending is detected at step 308, then the power supply is controlled toarc ending state 310. Arc ending state 310 generally provides for a timedelay while the power supply is disabled and cannot re-enter the weldingstate. This prevents inadvertent re-striking of the arc by terminatingthe output power. The time delay is supplied by step 312, and is 200msec in the preferred embodiment. The arc end state may be implementedwith an arc end control circuit 109 (FIG. 1), which is preferably partof controller 210 and may be digital, analog, software, firmware, etc.

[0047] Step 308 is implemented with a novel control for a TIG weldingprocess (and the novel control can be used with other processes).Generally, the TIG process is ended when the output (arc) voltage risesabove a threshold, rather than when the current decreases. Specifically,in the preferred TIG embodiment, the process ends when the outputvoltage rises above 25 volts, and a delay of 50 msec may be used (oromitted). Then, at step 310 the arc is immediately terminated and thepower supply is disabled (preferably for a period of time). The featuresthat determine if the arc voltage has increased beyond a threshold arecalled an arc length detect circuit because the arc length may beinferred from the data and the voltage comparisons.

[0048] Arc end state, as used herein, includes a state where the outputvoltage and/or current of the welding power supply is quickly terminated(such that the arc does not whip across the work piece). It may include(but does not require) a delay before the arc can be re-struck.

[0049] The process returns to step 310 (the arc end state with fulloutput) until it is determined at step 312 that the desired delay haselapsed. After the delay has elapsed the process returns to the standbystate at step 302, where the converter output is disabled, and the arcextinguishes. Thus, the welding process is terminated quickly without anarc being drawn longer and longer until it gets so long it extinguishesitself.

[0050] Numerous modifications may be made to the present invention whichstill fall within the intended scope hereof. Thus, it should be apparentthat there has been provided in accordance with the present invention amethod and apparatus for welding that fully satisfies the objectives andadvantages set forth above. Although the invention has been described inconjunction with specific embodiments thereof, it is evident that manyalternatives, modifications and variations will be apparent to thoseskilled in the art. Accordingly, it is intended to embrace all suchalternatives, modifications and variations that fall within the spiritand broad scope of the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A welding power supplycomprising: a source of power; a controller, connected to the source ofpower; an output feedback circuit, connected to the controller; whereinthe controller includes a comparator connected to receive a signalindicative of a threshold and a signal responsive to the feedbackcircuit, and wherein the controller further includes a standby/weldingcontrol connected to an output of the comparator and responsive to thecomparator.
 2. The apparatus of claim 1, wherein the standby/weldingcontrol includes at least a portion of a digital circuit.
 3. Theapparatus of claim 2, wherein the digital circuit includes instructionsthat control the output voltage to a first level an initial time thecomparator has a first output and that control the output voltage to asecond level in the event the comparator has a second output.
 4. Theapparatus of claim 2, wherein the digital circuit includes amicroprocessor with instructions that control the output of the powersupply to a standby status in the event the comparator initially has afirst output and that control the output of the power supply to awelding status in the event the comparator has a second output.
 5. Theapparatus of claim 1, wherein the feedback circuit includes a voltagefeedback circuit.
 6. The apparatus of claim 5, wherein thestandby/welding control includes at least a portion of a digitalcircuit.
 7. The apparatus of claim 6, wherein the digital circuitincludes a microprocessor with instructions that control the outputvoltage to a first level in the event the signal indicative of thethreshold initially is less than the signal responsive to the feedbackcircuit, and that control the output voltage to a second level in theevent the signal indicative of the threshold and the signal responsiveto the feedback circuit, wherein the first level is less than the secondlevel.
 8. The apparatus of claim 6, wherein the digital circuit includesa microprocessor with instructions that control the output of the powersupply to a standby status in the event the signal indicative of thethreshold is less than the signal responsive to the feedback circuit,and control the output of power supply to a welding status in the eventthe signal indicative of the threshold is greater than the signalresponsive to the feedback circuit.
 9. The apparatus of claim 1, whereinthe feedback circuit includes a current feedback circuit.
 10. A methodof providing welding power comprising: sensing an output parameter;comparing the sensed parameter to a threshold; and controlling a powersupply to be in a standby or welding state in response to thecomparison.
 11. The method of claim 10, wherein controlling includescontrolling the output voltage to a first level or a second level. 12.The method of claim 10, wherein comparing includes comparing an outputvoltage to the threshold.
 13. The method of claim 12, whereincontrolling includes controlling the output voltage to a first level ora second level.
 14. The method of claim 10, wherein comparing includescomparing an output current to the threshold.
 15. A welding power supplycomprising: means for sensing an output parameter; means for comparingthe sensed parameter to a threshold, connected to the means for sensing;and means for controlling a power source to be in a standby or weldingstate in response to the comparison, connected to the means forcomparing.
 16. The apparatus of claim 15, wherein the means forcontrolling includes means for controlling the output voltage to a firstlevel or a second level in response to the comparison.
 17. The apparatusof claim 16, wherein the means for comparing includes means forcomparing an output voltage to the threshold.
 18. The apparatus of claim17 wherein the means for controlling includes means for controlling theoutput voltage to a first level or a second level.
 19. The apparatus ofclaim 15 wherein the means for comparing includes means for comparing anoutput current to the threshold.
 20. A welding power supply comprising:means for providing power; means for controlling the means for providingpower, connected to the means for providing power; means for providingfeedback, connected to the means for controlling; wherein the means forcontrolling includes means for comparing a signal indicative of athreshold and a signal responsive to the means for providing feedback,and wherein the means for controlling further includes means forselecting one of a standby state and a welding state in response to themeans for comparing.
 21. The apparatus of claim 20, wherein the meansfor providing feedback includes a means for providing voltage feedback.22. The apparatus of claim 20, wherein the means for providing feedbackincludes a means for providing current feedback.
 23. A method ofstarting a welding process comprising: sensing an output parameter;determining if an electrode touches a workpiece; controlling a powersupply to be in a standby state if the electrode is initially nottouching the workpiece; and controlling the power supply to be in awelding state if the electrode is touching the workpiece.
 24. The methodof claim 23, wherein the welding state includes a first output voltageand the standby state includes a second output voltage, wherein thefirst output voltage is greater than the second output voltage.
 25. Themethod of claim 22, wherein determining includes comparing an outputvoltage to a threshold.
 26. An apparatus for starting a welding process,comprising: means for sensing an output parameter; means determining ifan electrode touches a workpiece, connected to the means for sensing;and means for controlling a power supply to be in a standby state if theelectrode is not touching the workpiece and to be in a welding state ifthe electrode is touching the workpiece, connected to the means fordetermining.
 27. The apparatus of claim 26, wherein the means fordetermining includes means for comparing an output voltage to athreshold.
 28. An apparatus for starting a welding process, comprising:an output feedback circuit; a short-circuit detect circuit, responsiveto the feedback circuit; a controller, connected to the short-circuitdetect circuit, wherein the controller includes a standby state output,provided if the electrode is not initially touching the workpiece andfurther includes a welding state output, provided if the electrode istouching the workpiece.
 29. The apparatus of claim 26, wherein thecontroller includes a digital circuit.
 30. The apparatus of claim 26,wherein the digital circuit includes a microprocessor, and themicroprocessor further is at least a portion of the short-circuit detectcircuit.
 31. The apparatus of claim 30 further including an arc lengthdetect circuit, responsive to the feedback circuit, and wherein thecontroller is connected to the arc length detect circuit and includes anarc end state output provided if the arc length is greater than a secondthreshold.
 32. A welding power supply comprising: a source of power; acontroller, connected to the source of power; an output feedbackcircuit, connected to the controller; wherein the controller includes acomparator connected to receive a signal indicative of a threshold and asignal responsive to the feedback circuit, and wherein the controllerfurther includes an arc end control connected to an output of thecomparator and responsive to the comparator.